Feature Article

Track of the Coastal Glider deployed for the Korea Institute of Ocean Science and Technology.

Gliders are well-known in the industry for providing long-duration sensing platforms. The original, or legacy, gliders (Slocum, Seaglider and Spray) were developed under an initial Office of Naval Research (ONR) grant in the late 1990s and early 2000s, with a common size and operating envelope based on the initial criteria that they be two-man portable (i.e., lighter than 50 kilograms) and able to operate in the open ocean.
While these requirements are suitable for a range of applications, they impose unrealistic constraints that make legacy gliders difficult or impossible to use in some environments, particularly coastal areas with large density gradients and strong currents. For example, these gliders cannot adaptively ballast the unit for continuous operation from freshwater to seawater without manual intervention or continuously vary the speed over a wide range.

To bring these characteristics to gliders, Exocetus Development LLC (Anchorage, Alaska) has been modifying ANT LLC's (Anchorage) Littoral Glider, now called the Coastal Glider, since purchasing its assets, intellectual property and manufacturing technology in October. The ANT glider had been developed with ONR funding during the past six years.

The Coastal Glider is undergoing modifications, which includes the installation of a dedicated science computer and improved communications and survivability.

Glider Components
Rated to 200 meters depth, the Coastal Glider is capable of self-ballasting from fresh- to ocean water and has a variable speed capability to handle nearshore currents up to 2 knots. During initial design stages, potential users requested a maximum speed up to 5 knots, but the general consensus was that 2 knots would be sufficient.

Buoyancy Engine. The requirement for adaptive ballasting meant that the buoyancy engine (BE) volume be at least 3 percent of the vehicle volume. After the glider's hydrodynamic design was completed, it was determined that a 2 percent variation was needed. Thus, the BE volume of 5 liters represents 5 percent of the vehicle volume.

Several types of BE drive mechanisms were investigated. The simplest mechanism for driving the BE is a ball-screw arrangement. Ball screws, however, are subject to being back driven if there is no locking mechanism or locking current. These options would add excessively to the complexity or the power consumption, so the ball drive was eliminated. Alternatively, an Acme drive could have generated the necessary high drive forces without the back-drive issue, but its efficiency is very low. For these reasons, a hydraulic system was selected.

Hydro-Leduc's (Azerailles, France) Model PB32.5 microhydraulic pump was investigated, as it had been successfully used in profiling buoys and other gliders. The best efficiency point of this pump occurs at an operating pressure of 34.5 megapascals. This is significantly higher than the 2-megapascal water pressure at the maximum depth of 200 meters. A reverse-amplifier BE system was designed so that, at maximum depth, the BE pump operates at the best efficiency point. Coupling the pump to a high-efficiency DC motor resulted in a BE efficiency of 70 percent.

Pitch and Roll System. The pitch and roll components were selected such that a single mass (the battery) could be used for both pitch and roll control. This was accomplished by selecting a high-efficiency commercial off-the-shelf (COTS) linear actuator for the pitch system with an integral linear potentiometer for position feedback. This actuator was connected to an adaptor plate, which was in turn connected to the battery mass. The linear actuator is equipped with an integral slip clutch to prevent damage in case it is overdriven.

The battery mass was designed so the center of gravity of the battery was 2 centimeters below the glider's centerline. The layout of other components was designed so the glider's center of gravity rested on its centerline.

The roll actuator is also a COTS DC motor, which is mounted on the adaptor plate and, thus, moves with the battery mass. The roll system is connected to the battery mass by a drive chain, and measurement of the battery roll is affected by the use of a separate potentiometer engaged with the chain. Limit switches are also integrated into the adaptor plate to prevent damage if the roll system is overdriven.

All of the drive components (BE pump, BE valve, pitch motor and roll motor) are located in the front of the glider to keep the electrically noisy equipment away from the communications equipment, navigation equipment and science equipment in the aft electronics bay.

The arrangement of these components dictated a larger diameter than is found on other gliders. The outside diameter of the main hull measures 0.32 meters for compatibility with equipment for handling small-size standard torpedoes.

Battery and Power System. The glider runs on a lithium battery, with all electronics designed to be powered by 18 to 32 volts DC.

Power use in a glider is very irregular. The glider will go for long periods of time on the descent or ascent using very little power. Most of the motive power is used in a few seconds at the bottom inflection. This type of large draw is difficult for batteries to provide and tends to shorten battery life.

For this reason, an ultracapacitor pack was installed, from which all of the main actuators are powered. This also helps to decouple the motors from the more sensitive electronic equipment. The ultracapacitors are located below the pitch, roll and internal BE components. To continue this article please click here.

Ray Mahr. Jr. is the co-founder of Exocetus Development LLC. He has extensive experience in the design, development and sales of instrumentation and equipment systems for oceanographic and underwater acoustics applications. As a former senior manager at three oceanographic instrumentation companies, he directed implementation of new oceanographic products and systems.

Joe Imlach was the chief mechanical engineer at ANT LLC and served as the principal investigator and program manager on the Littoral Glider project. He subsequently formed Exocetus Development LLC, which purchased the rights to the Littoral Glider product from ANT. He is currently president and chief technology officer of Exocetus.

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